So if I've understood correctly, a span marked as gd should be announced in English/Dutch/Russian or whatever the reader is using as "Scots Gaelic", and because it's most unlikely to have that language installed, it will then attempt to pronounce it as English/Dutch/Russian or whatever - yes?
Yes.

And I don't see how it could be wrong to add a lang attribute to an img tag. It says in the specs that it's a possibly attribute of the img element: http://www.w3.org/TR/html4/struct/objects.html#h-13.2 (under "Attributes defined elsewhere).